1. Field of the Invention
The present invention pertains to optical pulse stretchers, and, more particularly, to an optical pulse stretcher etalon.
2. Description of the Related Art
The increasing power and speed of many types of electronic systems has focused attention on optical technologies. Optical technologies are attractive for a variety of reasons that may vary depending on the type of technology. However, desirable characteristics typically include generally faster transmission and higher bandwidth of optical signals relative to electrical signals. Information is typically conveyed in optical signals in a series of light pulses, or xe2x80x9coptical pulse trains.xe2x80x9d Frequently, although not always, the optical pulses convey information digitally in 0xe2x80x2s and 1xe2x80x2s just as electrical pulses in electrical signals do.
Optical technologies are nevertheless not without their own problems. One problem arises from the fleeting nature of the light pulses. Demanding applications employ light pulses of very short duration, e.g., 1 ns or 1 ps, and future increases in capabilities are expected to push pulse widths even lower. Although the signals in optical systems are light pulses, the optical systems themselves employ opto-electronic components that are partly optical and partly electronic. Pulse widths of the magnitudes contemplated herein are difficult for the electronic side of the opto-electronic components to process in a timely fashion.
One solution to this dilemma is to xe2x80x9cstretchxe2x80x9d the light pulses. One common type of pulse stretcher is the xe2x80x9cetalon.xe2x80x9d An etalon is basically a cavity bounded by two reflective surfaces. One surface is essentially completely reflective, e.g., a mirror. The other surface is partially reflective, i.e., part of the optical signal will reflect off the surface while a portion of the optical signal propagates through the surface. Thus, an optical signal is introduced into the cavity and impinges upon the partially reflective surface first. A portion of the optical signal propagates through the partially reflective surface. A portion also reflects off the partially reflective surface to the fully reflective surface, which then again reflects from the fully reflective portion. The twice reflected portion then impinges upon the partially reflective surface, whereupon the process repeats. The pulse portions propagating through the partially reflective surface are then collected and combined to create a xe2x80x9ccompositexe2x80x9d pulse.
This xe2x80x9ccompositexe2x80x9d pulse is xe2x80x9cstretched,xe2x80x9d i.e., of longer duration than the xe2x80x9cbasexe2x80x9d pulse that was originally directed into the etalon. This technique admirably produces wider, i.e., longer duration, pulses. The stretched pulses are easier for the electrical part of the opto-electronic components to handle and process. However, several drawbacks accompany these pulse stretching techniques.
One significant problem is information loss. For instance, the width of the pulse might carry certain information that can be useful. The spacing of the reflective surfaces in conventional etalons, however, is driven by considerations such as volume and size, as opposed to any characteristic of the light pulses. The characteristics of the resultant stretched pulse bear no relationship to the characteristics of the base pulse from which such information can be retrieved. Thus, meaningful information that may be conveyed by the width of the base pulse is lost.
Another significant problem impacts the detector that processes the stretched pulses. The stretched pulse typically presents abrupt changes in intensity levels, e.g. from xe2x80x9coffxe2x80x9d to xe2x80x9con,xe2x80x9d with very short transition periods. These abrupt transitions can generate a phenomenon known as xe2x80x9cringingxe2x80x9d in the detector""s electronics. The detector""s electronics consequently must be designed to deal with this phenomenon, which complicates the electronics and can increase the space needed for the electronics within the optical system. Complexity is generally proportional to the rate of failure. In some high performance applications, space is at a premium. The need to compensate for ringing can therefore be highly undesirable in some applications.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.
The invention, in its various aspect, embodiments, and implementations, is a method and apparatus for stretching a pulse, shaping a stretched pulse, and modeling a stretched and/or shaped pulse.
In a first aspect, the invention includes an etalon comprising a port, a partially reflective surface, and a fully reflective surface. A base pulse may be introduced through the port whereupon it impinges upon the partially reflective surface. The fully reflective surface is spaced apart from the partially reflective surface a distance proportional to the width of the base pulse in operative relationship to the partially reflective surface. In various embodiments, this aspect includes such an etalon employed as a pulse stretcher in an optical system.
In a second aspect, the invention includes a method for shaping an optical pulse. The method comprises introducing a base pulse into an etalon, the etalon including a fully reflective surface spaced apart from a partially reflective surface a distance proportional to the width of the base pulse; collecting a plurality of portions of the base pulse propagating from the etalon; and combining the plurality of portions to generate a stretched pulse whose width is proportional to the width of the base pulse.
In a third aspect, the invention includes a method for modeling an optical pulse stretcher. The method comprises assigning a transmission factor value to each one of a plurality of taps; assigning a reflection factor value to each one of the taps, excepting only one tap; and assigning a transport delay for each tap to which a reflection factor value was assigned, wherein the transport delay is proportional to the width of a base pulse. This third aspect also includes, in its variants, a program storage medium encoded with instructions that perform such a method when executed by a computer and a computer programmed to perform such a method.